1. Field of the Invention
This invention relates to D/A conversion apparatus and method of a floating type which perform level conversion of input digital data by different conversion factors, converts the resulting data into analog data, then restores the analog data to an original level of the input digital data, and carries out addition of the analog data, thereby achieving an increased dynamic range of the reproduced sound.
2. Prior Art
In recent years, the conversion accuracy of A/D converters has been improved by virtue of delta-sigma modulators of higher order, and with this improvement, there is an increasing demand for further enhancement in the resolution and dynamic range of D/A converters. To meet the demand, a D/A conversion apparatus of a floating type has been conventionally developed which uses a D/A converter (hereinafter referred to as the xe2x80x9cDACxe2x80x9d) having a limited number of bits for conversion, and is capable of realizing a resolution and a dynamic range exceeding respective levels attainable by the limited number of bits for conversion. In this type of converter, when an N-bit (e.g. 20-bit) DAC is used for carrying out D/A conversion of M-bit (M greater than N: e.g. 24-bit) digital data, if the digital data has P bits (Mxe2x89xa7P greater than N) as effective bits, the digital data is directly subjected to D/A conversion without being further processed, and Mxe2x88x92N less significant bits (e.g. four less significant bits) are truncated. On the other hand, if the input level of the digital data lowers so that the effective word length of the same is reduced to Pxe2x80x2 bits (Pxe2x80x2xe2x89xa7N), the digital data is converted into data obtained by multiplying the same by a conversion factor of 2Mxe2x88x92N, i.e. by shifting the original data toward MSB (most significant bit) by Mxe2x88x92N bits so that the Mxe2x88x92N less significant bits have a value of zero, and then the resulting level-converted data is subjected to D/A conversion. Whether input digital data is to be subjected to D/A conversion without being further processed or after being multiplied by the conversion factor of 2Mxe2x88x92N is determined depending on whether an overflow of data occurs when the input digital data is shifted by Mxe2x88x92N bits.
In the D/A conversion apparatus constructed as above, when input data has P significant bits as effective bits, the length of word or bits for conversion is sufficiently large, so that the effect of the truncation of the Mxe2x88x92N less significant bits is almost negligible (even if a problem occurs due to the truncation, it can be solved e.g. by additionally carrying out dithering as required). On the other hand, when the effective bit length of input data is Pxe2x80x2 bits, the data is multiplied by the conversion factor of 2Mxe2x88x92N, and the Mxe2x88x92N less significant bits thereof are truncated during the D/A conversion. Therefore, in this case, Mxe2x88x92N less significant bits of the data which would be truncated if the data were not multiplied by the conversion factor of 2Mxe2x88x92N can be effectively D/A converted, whereby an increased resolution and an increased dynamic range are achieved. In the latter case, however, since an analog signal output from the DAC also has a magnitude multiplied by 2Mxe2x88x92N, it is required to carry out a level adjustment by multiplying the analog output by 1/2Mxe2x88x92N.
The D/A conversion apparatus of the floating type constructed as above includes one which employs a single DAC and the gain of an amplifier that amplifies an output from the single DAC is switched according to the conversion factor by which the level of input digital data is converted, and another which employs a plurality of DAC""s that perform D/A conversion of plural pieces of digital data obtained through level conversion of input digital data by respective different conversion factors, and one of the outputs from the DAC""s which has been subjected to the level conversion by the most appropriate conversion factor is selected (Japanese Patent Publication (Kokoku) No. 7-93579).
However, according to the former floating-type D/A conversion apparatus, since it is required to switch the gain of the analog amplifier instantaneously according to the level of the digital data, the output of the amplifier cannot follow up the switching, or DC offset of the amplifier can fluctuate, which can produce untoward noise which is audible. The latter floating-type D/A conversion apparatus also switches between analog signals output from the DAC""s, so that transient noise occurs upon the switching. These problems are extremely serious particularly when the resolution of digital data to be subjected to D/A conversion covers even a low noise range e.g. an SN ratio of 120 to 140 dB which can be conventionally realized only by analog circuitry.
To solve these problems, the present assignee has already proposed, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 11-308109, a D/A conversion apparatus of an advanced cross-fading algorithm type that is capable of effectively preventing generation of noise upon the switching between DAC""s, by cross-fading the digital data in advance of the switching. This D/A conversion apparatus generates only a very small amount of noise upon the switching between the DAC""s and is therefore capable of realizing a high-accuracy D/A conversion, as well as achieving an improved dynamic range.
In this D/A conversion apparatus, the advanced cross-fading operation is made possible by provision of a delay memory for delaying input digital data by a predetermined time period. To reduce the level of noise upon the switching between the DAC""s, it is preferred that the cross-fading is carried out at a gentle cross-fading rate (by a small cross-fading step i.e., over a large number of cross-fading steps) or over a sufficiently long time period. However, to realize such a gentle advanced cross-fading without causing overflow and clipping of data even when the level of the digital data steeply changes, the delay memory is required to provide a large amount of delay, which leads to an increased size of hardware, and hence to an increased manufacturing cost. Further, such an increase in the amount of delay results in a perceivable auditory delay, particularly when the D/A conversion apparatus is applied to a mixer that processes data of live sounds, or the like.
It is an object of the present invention to provide a D/A conversion apparatus and a D/A conversion method of a floating type which are not only capable of low-noise and high-dynamic range D/A conversion but also capable of carrying out the advanced cross-fading at a cross-fading rate sufficiently gentle for practical use by using a delay memory having a small amount of delay.
To attain the above object, according to a first aspect of the invention, there is provided a D/A conversion apparatus comprising a digital signal processor that carries out level conversion of same input digital data by different conversion factors into a plurality of level-converted digital data, selects and outputs most appropriate data of the plurality of level-converted digital data based on a signal quality of each of the plurality of level-converted digital data, outputs other data of the plurality of level-converted digital data after attenuating the other data to or below a predetermined noise level, and switches between data previously selected as the most appropriate data and data newly selected as the most appropriate data by carrying out cross-fading between the previously selected data and the newly selected data, a plurality of D/A converters that carry out D/A conversion of the plurality of level-converted digital data output from the digital signal processor to respective analog signals and outputs the analog signals, and an analog adder device that carries out level conversion of the analog signals output from the plurality of D/A converters again based on respective corresponding ones of the conversion factors in a manner such that resulting analog signals have a level corresponding to a level of the input digital data, and then adds together all of the level-converted analog signals.
The D/A conversion apparatus according to the first aspect of the invention is characterized in that the digital signal processor comprises a cross-fading section that carries out cross-fading between the previously selected data and the newly selected data, a delay section that delays the input digital data by a predetermined amount of delay which is shorter than a time period required for completion of the cross-fading carried out at a predetermined cross-fading rate by the cross-fading section, and a cross-fading control section that controls the cross-fading section such that when an amplitude level of the input digital data exceeds a predetermined threshold value, the cross-fading section starts the cross-fading at the predetermined cross-fading rate between the previously selected data and the newly selected data which are delayed by the predetermined amount of delay by the delay section, and that when during the cross-fading a rate of change in the amplitude level of the input digital data, which is larger than a predetermined rate of change, is detected, the cross-fading section carries out the cross-fading at a cross-fading rate larger than the predetermined cross-fading rate in dependence on the rate of change in the amplitude level of the input digital data.
According to this D/A conversion apparatus, when the amplitude level of the input digital data is increasing or decreasing, cross-fading (advanced cross-fading) is carried out between level-converted digital data based on a predetermined amount of delay shorter than a time period required for completion of the cross-fading carried out at a predetermined cross-fading rate. When the amplitude level of the input digital data increases at a gentle rate, the cross-fading is completed in the time period required for completion of the cross-fading without causing an overflow or clipping of data. Further, it is determined whether or not that a rate of change in the amplitude level of the input digital data is larger than a predetermined rate, and when it is determined that the rate of change in the amplitude level of the input digital data is larger than the predetermined rate, the cross-fading rate is increased in dependence on the rate of change in the amplitude level, such that an overflow or clipping of selected level-converted digital data can be prevented.
According to this D/A conversion apparatus, while the cross-fading can be carried out at a sufficiently gentle cross-fading rate for prevention of noise due to gain discrepancies upon switching between gains, the amount of delay provided by the delay section can be sufficiently reduced. This makes it possible to reduce the capacity of the delay memory to simplify the construction of the apparatus and reduce the manufacturing cost. Further, the cross-fading coefficient can be changed in a manner adapted to a larger rate of change in the amplitude level of the input digital data. In this case, the noise-eliminating effect by the cross-fading is degraded. However, since the input signal itself is rapidly changing, the auditory masking effect makes the noise practically imperceptible.
To attain the above object, according to a second aspect of the invention, there is provided a D/A conversion apparatus comprising a digital signal processor that carries out level conversion of same input digital data by different conversion factors into a plurality of level-converted digital data, selects and outputs most appropriate data of the plurality of level-converted digital data based on a signal quality of each of the plurality of level-converted digital data, outputs other data of the plurality of level-converted digital data after attenuating the other data to or below a predetermined noise level, and switches between data previously selected as the most appropriate data and data newly selected as the most appropriate data by carrying out cross-fading between the previously selected data and the newly selected data, a plurality of D/A converters that carry out D/A conversion of the plurality of level-converted digital data output from the digital signal processor to respective analog signals and outputs the analog signals, and an analog adder device that carries out level conversion of the analog signals output from the plurality of D/A converters again based on respective corresponding ones of the conversion factors in a manner such that resulting analog signals have a level corresponding to a level of the input digital data, and then adds together all of the level-converted analog signals.
The D/A conversion apparatus according to the second aspect of the invention is characterized in that the digital signal processor comprises a cross-fading section that carries out cross-fading between the previously selected data and the newly selected data, a delay section that delays the input digital data by a predetermined amount of delay which is shorter than a time period required for completion of the cross-fading carried out at a first predetermined cross-fading rate by the cross-fading section, and a cross-fading control section that controls the cross-fading section such that when an amplitude level of the input digital data exceeds a first predetermined threshold value, the cross-fading section starts the cross-fading at the first predetermined cross-fading rate between the previously selected data and the newly selected data which are delayed by the predetermined amount of delay by the delay section, and that when during the cross-fading the amplitude level of the input digital data exceeds a second predetermined threshold value larger than the first predetermined threshold value, the cross-fading section carries out the cross-fading at a second predetermined cross-fading rate larger than the first predetermined cross-fading rate.
The D/A conversion apparatus according to the second aspect of the invention provides the same effects as obtained by the D/A conversion apparatus according to the first aspect of the invention.
Preferably, the second predetermined threshold value is set to a limit of the amplitude level of the input digital data beyond which an overflow occurs when level conversion is carried out on the input digital data by a corresponding one of the different conversion factors, and the second cross-fading rate is set such that the cross-fading is completed in a time period corresponding to the predetermined amount of delay provided by the delay section.
According to this preferred embodiment, it is possible to secure a longer time period for the cross-fading at the first cross-fading rate, and at the same time, the second cross-fading rate can be set to a slowest rate within the limit defined by the predetermined amount of delay.
More preferably, the cross-fading is carried out using a cross-fading coefficient, the time period required for completion of the cross-fading carried out at the first predetermined cross-fading rate is a time period required for processing of N1 samples of the input digital data, and the predetermined amount of delay provided by the delay section corresponds to a time period required for processing of N2 samples of the input digital data (N1 greater than N2). The cross-fading coefficient is incremented/decremented by 1/N1 for each step of the cross-fading, after the amplitude level of the input digital data exceeds the first threshold value and before the amplitude level of the input digital data exceeds the second predetermined threshold value, and the cross-fading coefficient is incremented/decremented by (1xe2x88x92K)/N2 for each step of the cross-fading, after the amplitude level of the input digital data exceeds the second predetermined threshold value, wherein K represents a value of the cross-fading coefficient assumed at a time point the amplitude level of the input digital data exceeds the second threshold value.
To attain the above object, according to a third aspect of the invention, there is provided a D/A conversion method comprising the steps of: delaying input digital data by a predetermined amount of delay which is shorter than a time period required for completion of cross-fading carried out at a predetermined cross-fading rate; carrying out level conversion of the delayed input digital data by different conversion factors into a plurality of level-converted digital data; selecting and outputting most appropriate data of the plurality of level-converted digital data based on a signal quality of each of the plurality of level-converted digital data, and outputting other data of the plurality of level-converted digital data after attenuating the other data to or below a predetermined noise level; switching between data previously selected as the most appropriate data of the plurality of level-converted digital data and data newly selected as the most appropriate data by carrying out the cross-fading between the previously selected data and the newly selected data; carrying out D/A conversion of the plurality of level-converted digital data to respective analog signals and outputting the analog signals; and carrying out level conversion of the analog signals again based on respective corresponding ones of the conversion factors in a manner such that resulting analog signals have a level corresponding to a level of the input digital data, and then adding together all of the level-converted analog signals; wherein when an amplitude level of the input digital data exceeds a predetermined threshold value, the cross-fading is started between the previously selected data and the newly selected data which are delayed by the predetermined amount of delay, and when during the cross-fading a rate of change in the amplitude level of the input digital data, which is larger than a predetermined rate of change, is detected, the cross-fading is carried out at a cross-fading rate larger than the predetermined cross-fading rate in dependence on the rate of change in the amplitude level of the input digital data.
The D/A conversion method according to the third aspect of the invention provides the same effects as obtained by the D/A conversion apparatus according to the first aspect of the invention.
To attain the above object, according to a fourth aspect of the invention, there is provided a D/A conversion method comprising the steps of: delaying input digital data by a predetermined amount of delay which is shorter than a time period required for completion of cross-fading carried out at a first predetermined cross-fading rate; carrying out level conversion of the delayed input digital data by different conversion factors into a plurality of level-converted digital data; selecting and outputting most appropriate data of the plurality of level-converted digital data based on a signal quality of each of the plurality of level-converted digital data, and outputting other data of the plurality of level-converted digital data after attenuating the other data to or below a predetermined noise level; switching between data previously selected as the most appropriate data of the plurality of level-converted digital data and data newly selected as the most appropriate data by carrying out the cross-fading between the previously selected data and the newly selected data; carrying out D/A conversion of the plurality of level-converted digital data to respective analog signals and outputting the analog signals; and carrying out level conversion of the analog signals again based on respective corresponding ones of the conversion factors in a manner such that resulting analog signals have a level corresponding to a level of the input digital data, and then adding together all of the level-converted analog signals; wherein when an amplitude level of the input digital data exceeds a first predetermined threshold value, the cross-fading is started at the first predetermined cross-fading rate between the previously selected data and the newly selected data which are delayed by the predetermined amount of delay, and when during the cross-fading the amplitude level of the input digital data exceeds a second predetermined threshold value larger than the first predetermined threshold value, the cross-fading is carried out at a second predetermined cross-fading rate larger than the first predetermined cross-fading rate.
The D/A conversion method according to the fourth aspect of the invention provides the same effects as obtained by the D/A conversion apparatus according to the second aspect of the invention.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction of the accompanying drawings.